One of the most important and fundamental processes in biology is the division of cells mediated by the cell cycle. This process ensures the controlled production of subsequent generations of cells with defined biological function. It is a highly regulated phenomenon and responds to a diverse set of cellular signals both within the cell and from external sources. A complex network of tumor promoting and suppressing gene products are key components of this cellular signaling process. Over expression of the tumor-promoting components or the subsequent loss of the tumor-suppressing products will lead to unregulated cellular proliferation and the generation of tumors (Pardee, Science 246:603-608, 1989). Cyclin-dependent kinases play a key role in regulating the cell cycle machinery. These complexes consist of two components: a catalytic subunit (the kinase) and a regulatory subunit (the cyclin). To date, nine kinase subunits (cyclin-dependent kinase 1-9) have been identified along with several regulatory subunits (cyclins A-H, K, N, and T). Each kinase associates with a specific regulatory partner and together make up the active catalytic moiety. Each transition of the cell cycle is regulated by a particular cyclin-dependent kinase complex: G1/S by cyclin-dependent kinase2/cyclin E, cyclin-dependent kinase4/cyclin D1 and cyclin-dependent kinase6/cyclinD2; S/G2 by cyclin-dependent kinase2/cyclin A and cyclin-dependent kinase1/cyclin A; G2/M by cyclin-dependent kinase1/cyclinD. The coordinated activity of these kinases guides the individual cells through the replication process and ensures the vitality of each subsequent generation (Sherr, Cell 73:1059-1065, 1993; Draetta, Trends Biochem. Sci. 15:378-382, 1990).
An increasing body of evidence has shown a link between tumor development and cyclin-dependent kinase related malfunctions. Overexpression of the cyclin regulatory proteins and subsequent kinase hyperactivity have been linked to several types of cancers (Jiang, Proc. Natl. Acad. Sci. USA 90:9026-9030, 1993; Wang, Nature 343:555-557, 1990). More recently, endogenous, highly specific protein inhibitors of cyclin-dependent kinases were found to have a major affect on cellular proliferation (Kamb et al., Science 264:436-440, 1994; Beach, Nature 336:701-704, 1993). These inhibitors include p16INK4 (an inhibitor of cyclin-dependent kinase4/D1), p21CIP1 (a general cyclin-dependent kinase inhibitor), and p27KIP1 (a specific cyclin-dependent kinase2/E inhibitor). A recent crystal structure of p27 bound to cyclin-dependent kinase2/A revealed how these proteins effectively inhibit the kinase activity through multiple interactions with the cyclin-dependent kinase complex (Pavletich, Nature 382:325-331, 1996). These proteins help to regulate the cell cycle through specific interactions with their corresponding cyclin-dependent kinase complexes. Cells deficient in these inhibitors are prone to unregulated growth and tumor formation. This body of evidence has led to an intense search for small molecule inhibitors of the cdk family as therapeutic agents.